Hugo docs: Fix {{site.baseurl}} links to use relative internal links instead

[bernhardkaindl]

For links to internal pages, {{site.baseurl}} (without the leading .) does not
work in current Hugo, and it is better to replace it by using relative links:

{{.Site.BaseURL}} is replaced in the rendered output, but it doesn't work when
viewing and editing the files using Markdown editors and viewers like on GitHub/Lab
as well as IDEs with improved support for Markdown editing(in editor and preview).

As the relative links work in all cases like in the editors and previews,
fix those (currently broken) links to use the relative links instead.

While at those, also replace the link name "here" with a more specific link name.

PS: The only place were {{.Site.BaseURL}} or similar is needed is the link in
the logo.html template for img="{{.Site.BaseURL}}/images/xapi-project.png"